This invention relates generally to a process which requires recovery of a desirably white reaction product of a cyanuric halide with an amine reactant (white being indicative of the product's high purity), in high yield. The desired tri-substituted triazine compound must be white enough to meet industry standards because the compound is a prized stabilizer for a variety of synthetic resinous materials, and the market-place demands that the tri-substituted triazine not contribute any unwanted color to the stabilized material. The reason for requiring a high yield is that the amine used to make the tri-substitution is so costly that the process would be uneconomical if the yield was less than about 90%.
More specifically this invention relates to a process for producing a high purity (at least 97% pure) tri-substituted triazine, in which the first step comprises substituting each of three chlorine (or other halogen) atoms on a tri-halo-s-triazine, specifically cyanuric chloride, with a polysubstituted piperazine, polysubstituted piperazin-2-one, polysubstituted 1,4-diazacycloheptane, polysubstituted 1,4-diazacycloheptan-2-one, or polysubstituted piperidine (each of which substituents are referred to by the acronym "PSP" for brevity). The desired tri-substituted triazine, referred to as PIP-T for brevity, therefore has a PSP substituent at each of the 2, 4 and 6 positions of the triazine ring.
In a conventional process for making the PIP-T, finely divided solid cyanuric chloride is reacted with the amine from which the PSP substituent is derived (this amine is referred to herein as "the PSP amine", or simply "the amine") in a large excess of solvent with little regard to how much is required to maintain a saturated solution of the components in solution. More solvent was used than is required to dissolve the solid cyanuric chloride and the PSP amine, with the expectation that the solvent will provide the medium in which the reaction may proceed with less restraint than if no solvent was used. The solvent provides a liquid phase in which the exothermic reaction proceeds controllably, and, in as little time as is safely practical. It will be appreciated that another cyanuric halide such as cyanuric bromide could also be used, but from a commercial point of view, only cyanuric chloride is practical.
These considerations dictated that such a solvent be chosen to provide the dual function of furnishing a relatively large heat sink from which, in turn, heat could be released to a cooling medium through the internal cooling coils of a reactor, or its jacket. Therefore, in the earliest process, the amine reactant was dissolved in a large excess of an alkylbenzene solvent; a strong base, such as an alkali metal hydroxide e.g. sodium hydroxide, and the cyanuric halide, usually cyanuric chloride, were then added gradually, both preferably as finely divided solids, to the solution. The base provided a catalytic function, and, as the reaction proceeded, chloride ions were generated (from the cyanuric chloride) forming HCl acid which was promptly neutralized by the NaOH. The reaction mass in the reactor was heated to conduct the reaction, then cooled after completion of the reaction, by a suitable heat exchange means (such as a cooling jacket on the reactor, or an internal cooling coil). The desired tri-substituted triazine was then recovered from the reaction mass by a conventional "work-up" in which the sodium chloride was washed away in hot water, and the solvent distilled from the toluene solution. This process is referred to as the "solvent process" disclosed in copending patent application Ser. No. 526,194 filed May 21, 1990 to Son et al.
Operation of this "solvent process" suffered from the serious disadvantage of corrosion of the reactor due to the presence of hot caustic and the generation of chloride ions under operating conditions which required a high temperature, well above the boiling point of the solvent at atmospheric pressure. The pressure generated in the reactor (which was related to the solvent used, and the amount of water present, if any), and the related high temperature (about 175.degree. C.-200.degree. C.) were required because of the difficulty of adding the third (and last) PSP substituent on the triazine ring. When an aqueous solution of concentrated NaOH (rather than solid NaOH) is used to facilitate its addition to the reactor, the contribution of the vapor pressure of the water exacerbates the problem of coping with pressure generated in the reactor.
Corrosion is a key contributor to "color" of the product. Therefore, not only is the profitability of operating a pressurized reactor at elevated temperature greatly diminished by the corrosion problem, but also the marketability of the PIP-T product. Unacceptably colored product which is not generally marketable has a "melt absorptivity" greater than 3.5 Ml/gm.cm determined as described hereinafter.
It will now be appreciated why it is difficult to add the third PSP substituent (referred to as the "last PSP substituent") under temperature and pressure conditions conducive to making substantially pure product, without incurring an unacceptable level of degradation of the product. Because the catalyst used in the prior art processes was caustic, practicality dictated the use of aqueous NaOH, but because the onus of marketability of the product was not then a consideration, the prior art failed to emphasize that degraded product is unmarketable if it is colored.
Even had it not been so desirable to add aqueous concentrated NaOH to the reactor in the solvent process, the problems of obtaining large quantities of anhydrous toluene, and maintaining operation of the system under truly anhydrous conditions was impractical.
From an appraisal of the foregoing facts it will now be evident that the problem to be solved has three distinct facets, namely (1) to make the desirably white tri-substituted triazine, (2) to control the corrosion of the reactor to a tolerable level, and (3) to recover and separate both the PIP-T product and the unreacted PSP amine without losing so much of either as to make the process uneconomical.
What is not evident from the foregoing clear enunciation of the problems to be solved, is that one must meet the requirements of marketability and economic production despite the impracticality of maintaining an anhydrous reaction system in a large reactor, say at least 100 gallons, such as is used for commercial preparation of the PIP-T stabilizer. The presence of a trace of moisture coupled with the unavoidable generation of chloride ions during the reaction, causes such severe corrosion even in a stainless steel reactor that not only is the product formed colored, but the reactor is damaged.
To cope with the problems of purity, yield and corrosion in the solventless process without using an Inconel or other uneconomical special alloy reactor, we devised a process, not long ago, in which neither a solvent (such as toluene) nor strong base (sodium hydroxide) catalyst was used. In this process, described in copending U.S. patent application Ser. No. 07/364,342 filed June 9, 1989, and incorporated by reference thereto as if fully set forth herein, a large excess of substantially anhydrous PSP amine is reacted with solid cyanuric chloride, the excess of amine used being in the range from 2 to 10 times the theoretical amount required to displace each of the Cl atoms of the cyanuric chloride, with the result that the last substituent was made in less than 12 hr.
That portion of the PSP over stoichiometric which was not used to provide the three PSP substituents, reacted with the HCl formed during the reaction, forming a PSP amine.Hcl salt (hereafter the "salt"). An alkanol (methanol) and concentrated aqueous solution of base (NaOH) was then added to neutralize the PSP amine.HCl, and form NaCl. The metered amount of methanol and water added, allowed at least 90% of the NaCl to be precipitated and filtered so as to leave the PIP-T solid which could be separated from the filtrate.
This filtrate had the peculiar property of being able to deliver white solid PIP-T particles when more water was added to the filtrate. The PIP-T particles could then be recovered, as could the excess amine reactant, methanol and water. This process is referred to as the "solventless process" because no solvent was used during the reaction in which each of the three PSP substitutions was made.
The foregoing solventless process, however, relied upon essentially complete separation of the PIP-T product from the PSP amine, and, recovery of the excess PSP amine for its economic viability, criteria which were not easily met under the conditions of that process. Moreover, because it was neither practical to obtain large quantities of reactant amine in an anhydrous state, nor to operate such an anhydrous reaction system on a commercial scale because traces of moisture were always present, the reactor was still prey (though to a lesser extent than in the solvent system) to corrosion.
In the relatively recent aforementioned "solvent" process we encountered the novel problem of unexpectedly severe corrosion even in a Hastelloy reactor. In the even more recent "solventless" process we tried, with some success, to solve the twin problems of separating the product PIP-T we made, and to recover the excess PSP amine we used. Since economics mandate the use of a stainless steel or glass-lined reactor (if a carbon steel one is impractical), along with effective recovery of essentially all the PIP-T made, not to mention the PSP amine, we were driven to explore still other routes to a fast and efficient process which substantially avoided the problems of both the "solvent" and the "solventless" processes.
It is this exploration which opened the door to the discovery that, in a limited amount of alkylbenzene solvent, preferably less by weight than the amount of amine used in the reaction, the PSP amine.HCl salt formed during the reaction, appears to speed up the reaction. Despite the relatively short period of reaction for the solventless process compared to that required for the solvent process, this "in situ catalytic effect" of the PSP amine.HCl salt went unrecognized in the solventless process, because of the large excess of amine present there. It was because of this in situ catalytic effect, no caustic catalyst was then, nor is now, required.
By "excess" of amine we refer to more than is required over theoretical to make the tri-substituted product. 3 mols of amine are theoretically required to make one mol of the product with one mol of cyanuric chloride. It takes an extra mol of amine (over theoretical to make the product) to react with the chloride ions generated. Thus it takes a minimum of 6 mols of amine to make 1 mol of product with our process. To ensure that all the chloride ions are reacted we use additional amine but less than 2 mols of amine for each chloride ion, so that we use less than 12 moles of amine for each mol of product made in the present process, referred to as the "hybrid" process.
Each of the several subsequent steps required for the work-up of the product, and recovery of the excess of amine deliberately used in our hybrid process (so termed for reasons which will now be more apparent), takes advantage of the peculiar physical characteristics of the solubility of each of the reactants and the reaction products. The ability to do this successfully, provides a solution to problems which adversely affected the economics of both the solvent and the solventless processes. Such steps, together provide a highly effective hybrid process for making the PIP-T product.
With particular regard to the effects of corrosion even in a stainless steel reactor, it will be recognized that, economics dictate that the presence of moisture which is responsible for such corrosion, can only be minimized, not excluded. Because of the relatively much smaller quantities of PSP amine (relative to the solventless process) and toluene (relative to the solvent process) required in the hybrid process, using a stainless steel reactor becomes economical. It will be also be recognized that, if a carbon steel reactor is used, the side reactions may be destructive and at the same time, generate much color. The color would have to be removed at a later stage, at great cost. Therefore, a carbon steel reactor is uneconomical, and either glass-lined or stainless steel reactors are chosen.